Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation.

Human transcallosal responses (TCRs) were elicited by focal magnetic coil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8-12.2 msec, a duration of 7-15 msec, and an amplitude which reached up to 20 microV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.